A conventional windmill with removable vane plates (shown in FIG. 1) comprises a plurality of vane plates arranged in only one row. This construction has a limited wind resistance and consequently a limited electric output from a generator connected to the windmill. If electric output is to be increased, the dimension of the wind vanes would have to be enlarged, involving danger of breakage caused by very strong wind. This conventional windmill is considered to have the following disadvantages:
1. Its electric power output is quite limited hardly meeting its cost.
2. If its wind vane is enlarged, its basic structure may not be strong enough to cope with a very strong wind, being subject to breaking or falling down.